1. Field of the Invention
This invention relates to a calibration device and calibrating system for optical catheters used in a catheter oximetry system, and more particularly, it relates to a calibrating device which may remain with the sealed and sterilized distal end of the catheter within a package while the proximal end of the catheter is plugged into a computer or processor in order to perform the calibrating operation.
2. Description of the Prior Art
A catheter oximetry system provides accurate, continuous, real-time measurement of mixed venous oxygen saturation using multiple wavelength reflection spectrophotometry. The color of red blood cells progressively changes from scarlet to purple as the amount of oxygen the red blood cells are carrying decreases. When light of different selected wavelengths illuminates the blood, the amount of light backscattered, or reflected, at each wavelength depends upon the color, and therefore, oxygen level of the blood. Careful choice of wavelengths in the transmittal light allows accurate measurement of oxygenated hemoglobin with minimal interference by other blood characteristics such as temperature, pH, and hematocrit.
Approximately 98% of the oxygen in the blood is chemically combined with hemoglobin in red blood cells. The absorption of red and infrared light substantially differs for oxygenated and deoxygenated hemoglobin, and it varies for different wavelengths of light within this red/infrared spectrum. Therefore, the relative amounts of oxygenated hemoglobin and deoxygenated hemoglobin in the blood can be determined by measuring the relative absorption of light at different selected wavelengths. The percentage of hemoglobin which is in the oxygenated form is defined as the oxygen saturation of the blood in the equation: ##EQU1## where HbO.sub.2 is the oxygenated hemoglobin concentration and Hb is the deoxygenated hemoglobin concentration.
A widely used catheter oximetry system consists of three basic components: (1) a disposable fiberoptic pulmonary artery catheter that has a distal end adapted to be inserted into a vein of a patient and that interfaces at its other end with (2) an optical module containing light emitting diodes, a photodetector and associated electronics, which in turn, interfaces with the electrical leads of (3) a computer-based instrument that performs all of the data processing and control functions with displays, alarms and associated read-out devices. The instrument and optical module may be reused many times with different patients, but the catheter is used only with a single patient during a single operation or monitoring process. Thus, the catheters are disposable and are arranged to be separately packaged in sealed aseptic packages each with a specially designed optical connector plug adapted to be plugged into the optical module when the catheter is ready for use.
Since the total amount of light reflected back from the blood under test during the catheter oximetry measurements is relatively low, and since variations in the manufacturing of the optical components (particularly the fiberoptics) create differences in transmission which affect the output readings, it is important that each catheter be separately calibrated immediately before it is used so as to relate the actual light intensities received from the sample under test to the unknown concentrations of the substances being quantified in the sample under test. This may be accomplished by initially measuring a given sample of blood with the catheter and then wholly independently measuring the same blood in the laboratory by a different technique in order to match the laboratory calculated actual oxygen saturation content with the instrument calculated content and adjusting the latter accordingly. Such a technique has the obvious disadvantage, however, that the time required for making the laboratory tests causes an undesirable delay between the time of catheter placement and the time at which the oxygen saturation measurements can be utilized with assurance of their correctness. In order to overcome this obvious disadvantage, various techniques have been proposed whereby the catheter is initially calibrated using a reference material such as suspensions of milk of magnesia combined with dyes or filters of various light reflective targets which the distal end of the catheter can be initially directed to and which have known reflectivity characteristics.
One method of initial catheter calibration which has found wide acceptance in the field is disclosed in U.S. Pat. No. 4,322,164 to Robert F. Shaw et al. Briefly, this method involves a reference block formed as a solid compliant mass having a plurality of light reflective particles embedded therein. This reference block is received within an enclosed tube, and, in the initial packaging of the catheter, the distal end thereof is inserted into the tube adjacent to but spaced from the reference block and gripped to restrain further movement. The reference block is then spring loaded but restrained by a releasable catch so that it can be released into resilient engagement with the end of the catheter at the time that the calibration measurements are made. Once the calibration readings have been obtained, the catheter can be pulled loose from the tube and reference block and placed in a patient for obtaining blood oxygen saturation measurements in the manner intended. The initial calibration readings are obtained with the reference block and catheter remaining in the package in a sealed and sterilized condition while the connector plug end of the catheter is connected to the optical module and oximetry processor.
While the aforedescribed catheter calibration scheme has met with considerable success, there have been some problems from time to time. Thus, it may be inconvenient for the doctor or nurse to perform the separate operation of releasing the reference block into engagement with the catheter tip, or, such operation may fail or expose the catheter to possible contamination prior to its actual time of use.